Cryogenic grinding is used for comminuting materials that are difficult to comminute, such as for example for producing powdered rubber from rubber granules. In this case, the coarse material to be ground is fed to a low-temperature embrittling device, for example in the form of a cooling screw, in which the coarse material is cooled down with a gas supplied in a liquid state, for example liquid nitrogen, and embrittled, so that it becomes friable. Then the material is charged to a mill and is ground. However, the ground material thus obtained contains many fineness fractions. In order to obtain a specific fraction of the ground material, generally the fine fraction, the ground material is then dried or heated and charged to a screening device, at which the fine fraction is sifted out. A corresponding apparatus and a corresponding process is known for example from U.S. Pat. No. 7,445,170.
It is disadvantageous there that the ground material has to be heated to allow it to be screened and that there is a downward limit on the achievable separation cut, since with finer mesh widths the screen deck becomes clogged. Also, if there are changes to the desired end fineness, the complete screen deck has to be exchanged.
For separating off a fine material fraction from a heterogeneous mixture of different grain sizes, so-called wind classifiers are also known. Examples of these wind classifiers can be found in U.S. Pat. No. 4,528,091 and in German patent application DE 10 2005 001 542 A. Wind classifiers have one or more classifier wheels arranged above a classifier chamber. Particles of the material to be classified are entrained by an upwardly directed air stream in the classifier chamber or, if the particles are too heavy, the particles are not entrained. Of the lighter particles that reach the classifier wheel, only those particles that are smaller than the upper grain limit defined by the classifier wheel pass the classifier wheel. The other particles fall back.
Mill/classifier combinations are also already known from non-cryogenic applications. In these cases, the ground material is blown through the mill to the classifier by large amounts of compressed air, which makes them appear to be unsuitable for use for cryogenic grinding, because of the temperature control of the mill and the associated pressure fluctuations in the gas/particle stream flowing out of the mill, and because it is not possible in this case for any amounts whatsoever of relatively warm compressed air to be blown in.
An example of such a classifier mill is known from U.S. Pat. No. 4,550,879, which shows a pencil-wheel hammer mill that is arranged upstream of the wind classifier and by which the charged material is blown upward through the mill into the classifier chamber of the wind classifier by compressed air. A classifier mill with a whirlwind mill and a classifier wheel arranged vertically above it is disclosed in German utility model specifications DE 910 96 08 U1 and DE 901 22 38 U1. Here too the ground material is transported to the classifier wheel by a lot of air that is additionally blown in.